In a wireless communications network, co-channel interference is one of the main factors limiting the capacity of the network. Co-channel interference is caused when the same frequency resources are used at the same time by two different transmitters. This may be compared to, for example, adjacent channel interference which is caused when leakage occurs from one frequency to another frequency due to imperfections in the receivers and transmitters. There exist several approaches for dealing with the co-channel interference in a multi-user scenario today. These approaches require different levels of coordination among the transmitters.
Interference alignment, IA, is one such technique that may be used to mitigate the co-channel interference between transmitter-receiver pairs; provided, however, that there are enough Degrees of Freedom, DoF, available. One example of when IA may normally be applied is when using a Multiple-Input Multiple-Output Interference Channel, MIMO-IC, wherein multiple transmitters simultaneously transmit data to their respective receivers.
IA involves multiple transmitters that attempts to align the caused interference to unintended receivers within a minimum-dimension subspace, so that at each receiver the remaining dimensions may be used for interference-free communication. This allows each receiver to eliminate all the interference by simply canceling everything that falls into this subspace. This is a rather general idea, in the sense that the radio signals may be aligned in any given dimension, such as time, frequency, or space. There are also several possible ways to specify and implement IA algorithms depending on the cost function to be optimized and on the level of coordination between network nodes.
In a conventional wireless communications network, only the intra-cell interference may sometimes be mitigated. Thus, when there exists coordination between different cells, such as, e.g. in a Coordinated Multi-Point, CoMP, network, the interference between cells, i.e. inter-cell interference, may be cancelled by using other techniques, such as, e.g. joint precoding. Joint precoding however requires a tight coordination between the network nodes and that the data streams are shared among the transmitters. However, when it is not possible to perform a joint precoding transmission, such as, e.g. in the case of loose coordination between the network nodes, then IA may be a suitable technique for cancelling the co-channel interference.
Also, with regard to interference limited scenarios, IA is especially good at low Signal-to-Interference-plus-Noise Ratios, SINRs, as it tends to be more robust than other interference cancellation techniques.
However, depending on the network configuration, which may be determined by e.g. the number of antenna elements both at the transmitter and at the receiver side, the number of simultaneous transmitter-receiver pairs, as well as, the number of data streams communicated between each transmitter-receiver pair, it may be impossible sometimes to completely cancel out the co-channel interference using IA. This is because the feasibility of using IA for a particular network configuration is directly related to solving a system of equations. The number of dimensions required at each network node may however easily become impracticable as the number of simultaneous users increases, that is, if too many simultaneous transmitter-receiver pairs are considered, then an exceedingly large number of antennas might be required at each network node.
Furthermore, given the IA feasibility conditions and assuming a certain network configuration, fewer data streams are typically transmitted when using IA coordinated transmissions as compared to transmissions without coordination. Another limitation of IA is that the technique achieves good results mainly for a high Signal-to-Noise Ratio, SNR, regime, where gains of the DoF affect the performance more significantly.
However, more importantly, it has been noted that using IA for interference mitigation may potentially result in lower link level throughputs, at least for some radio links in a wireless communications network.